The invention relates to a transmission, and more particularly to a transmission for wind power installations, with an input-side planetary stage comprised of at least two power-splitting planetary gears connected in parallel and at least one gear stage connected downstream of the planetary stage.
In a wind power installation, a transmission with a geared-up transmission ratio is typically installed in the force flux between a rotor, which transforms the wind energy into a rotary motion, and a generator, which transforms the rotary motion into electrical energy. It is highly desirable to have lightweight transmissions with the smallest possible dimensions, since the various modules of the wind power installation are primarily housed inside a capsule located on top of a tower. This requirement is in conflict with the trend to increasingly more powerful wind power installations, which then also require more efficient, i.e., heavier transmissions.
A lightweight transmission with a corresponding small size is known from WO 96/11338. The small weight-saving construction is realized by connecting two planetary gear stages in series. The planet carrier of the first input-side planet gear is connected to the rotor and forms the drive shaft for the gear. The hollow wheel of the first planetary gear is secured to the case, whereas the driven sun wheel is connected to an input-side planet carrier of the second planetary gear, whose hollow wheel is also secured to the case and whose sun wheel forms the driven (output) section of the entire gear. However, this series connection of two planetary gear stages, has the disadvantage that for larger size wind power installationsxe2x80x94in particular above 2 MWxe2x80x94the outside dimension of the transmission are too large even when a planetary gear arrangement is used. Simultaneously, the mass also reaches a critical value.
An alternative type of conventional transmissions of the type of interest has a input-side planetary gear stage, which is connected to the rotor. Unlike the conventional transmission with a second planetary gear stage described above, at least one gear stage with spur wheels is connected downstream which provides an additional up-conversion of the rotation speed of the rotor to the faster rotation speed desired for the generator. Due to the torque-rotation speed conditions, the input-side planetary gear stage in this design also determines the geometric dimensions of the entire gear, so that the problems caused by the large size and the corresponding large mass still persist.
An attempt to solve the aforedescribed problem included reducing the outside dimensions of the transmissionxe2x80x94in particular the diameterxe2x80x94and simultaneously the total mass by splitting the power in the input-side gear stage. The input power can be split by dividing the input-side planetary stage into at least two smaller planetary gears that are connected in parallel. Attempts have been made to equalize the torque between the two planetary gears after splitting the power, i.e., to split the power between the two planetary gears in a defined manner, by providing the engaging gear wheels with helical gearing to compensate for relative rotation of the output-side gear components of the planetary stage. The power can be equalized by providing the parallel-connected gear wheels of the first and second planetary gears of the input-side planetary stage with helical gears oriented in opposite directions. However, it has been observed in practice, that this still causes troublesome relative movements of the planetary wheels, because the large number of engagement points of the gear wheels of the planetary gear participating in the power transmission still causes persistent relative axial movements. This is caused primarily by manufacturing tolerances in the gear wheels. It is therefore difficult to achieve a satisfactory torque equalization in the planetary stage where the power is split.
It is therefore an object of the present invention to improve a power-splitting gear of the aforedescribed type, so that the torque can be precisely balanced even when using small lightweight gears.
According to one aspect of the invention, differential gear stage is connected downstream of the power-splitting planetary gears of a input-side planetary stage, so as to balance the load that was unequally split between the planetary gears as a consequence of the parallel connection.
Advantageously, the planetary stage includes exactly two planetary gears which are connected in parallel and arranged sequentially. This arrangement guarantees an effective power flux with a reasonable number of gear elements.
Advantageously, with this arrangement of the differential gear stage downstream of the input-side planetary stage, any relative movement of the gear elements due to manufacturing tolerances can be precisely neutralized. The differential gear stage equalizes a torque imbalance between the preferable two driven shafts of the planetary stage so as equally split the power. Even a smaller gear can then be operated with a defined power splitting, as implemented with the parallel connection of the power-splitting planetary gears of the input side planetary stage.
According to an improved embodiment of the invention, a sun shaft of the first planetary gear together with a sun shaft of the second planetary gear forms a coaxial hollow shaft assembly. The two planetary gears of the input side planetary stage can then also be positioned in coaxial disposition. On the output side, the corresponding sun wheels can also be arranged in coaxial and space-saving disposition.
The driven shaft of the gear can preferably be positioned with an axial offset from the drive shaft by using a spur gear stage, whereby means for controlling the rotor can be guided through a hollow drive shaft.
In a first preferred embodiment of the invention, the differential gear stage is formed as a so-called xe2x80x9cpassive differentialxe2x80x9d in the form of a differential planetary gear. The differential planetary gear splits the power equally between the two connected sun shafts of the symmetric planetary stage formed of planetary gears having equal dimensions. One sun shaft is hereby operatively connected with this sun wheel, whereas the other sun shaft is operatively connected with the hollow wheel of the differential gear stage. The planet carrier forms the driven (output) section. The passive differential gear stage can be arranged either coaxially with the sun shafts or with an axial offset from the sun shafts by way of an intermediate spur gear stage. The role of the passive differential in form of a differential planetary gear is to equalize the torque between the two planetary gears of the input-side planetary stage. Since the two planetary gears are constructed in an identical fashion, this embodiment is easy to manufacture and the replacement parts inventory can be kept low.
In a second preferred embodiment, the differential gear stage is formed as a so-called xe2x80x9cactive differentialxe2x80x9d in the form of a differential planetary gear. On one hand, the active differential splits the power equally between the two connected sun wheels of the planetary stage that is comprised of planetary gears with identical dimensions; on the other hand, the differential gear stage contributes to the overall transmission ratio of the gear through an asymmetric connection of the planetary gears of the planet stage, which will be described in more detail below. Because the active differential not only compensates the torque, but also contributes to the gear ratio of the gear, the size of the first planetary stage can be smaller than in the first embodiment, While still realizing the same gear ratio of the gear. Advantageously, integrating the functions of a differential with those of a gear also reduces the number of gear elements which in turn reduces the mass of the entire gear. This advantage is achieved primarily by the asymmetric connection of the input-side planetary stage.
According to a third embodiment of the invention, the differential gear stage can be implemented as a passive differential in form of a differential spur gear pair which is supported softly in the axial direction and has gear wheels with opposite helical gearing. Unlike the differential planetary gear according to the two preceding embodiments, the differential spur gear pair splits the power equally between the two connected sun shafts of the symmetrically constructed planetary stage that includes planetary gears having the identical dimensions. One sun shaft is operatively connected with one differential spur wheel of the spur gear pair, whereas the other sun shaft is operatively connected with the other differential spur wheel which has the opposite helical gearing. The differential spur gear pair can preferably be flexibly supported along the axial direction by a soft coupling element that is coaxially disposed between the differential spur gear pair and a coaxial spur wheel arranged on the output side. The soft coupling characteristic in the axial direction is preferably achieved with a coupling element having a body made of an elastomer.